Foldable member

ABSTRACT

A foldable member in the shape of a tube with at least one predetermined hinge area along the length of the tube, and a plurality of opposing elongated slots in the tube at the hinge area thereof forming separated longitudinal strips of tube material between the slots which fold when subjected to localized buckling forces. A collapsible structure constructed of such foldable members.

GOVERNMENT RIGHTS

This invention was made with U.S. Government support under Contract No.F29601-98-CO-108 awarded by the U.S. Air Force. The Government may havecertain rights in the subject invention.

RELATED PATENT APPLICATION

This application is a divisional application of a patent applicationentitled “Foldable Member” by the same inventor as the subjectapplication filed on Nov. 9, 1999.

FIELD OF INVENTION

This invention relates to a foldable boom, truss, or longeron member,collapsible trusses and other similar structures made of such members.

BACKGROUND OF INVENTION

Key optical components of large aperture, space based opticalinstruments may be deployed on orbit to provide an aperture large enoughto increase the resolution and optical performance by several orders ofmagnitude. The performance of such instruments depends on maintainingthe precision and stability of the deployed structural geometry towithin nanometers of an ideal shape. Nonlinear contact mechanics andfreedom in the components of deployed structures mean that deployedinstruments will have the capacity to change shape at the micron andnanometer level of resolution. Eliminating such nonlinearities as loadpath friction and freeplay would enable a deployed structure to be aslinear and precise as a monolithic block of material.

In most mechanically deployed structures, components are moved fromtheir stored positions into their final operational positions by sometype of actuator and then locked into place with a deployment latch. Forhigh precision structures, it is critical that the load paths and loadpredictable for the reliable operation of the instrument.

Existing deployable structure joints have several limitations thateither completely prevent them from being used in high precisiondeployable instruments or require complex analysis and additional launchmass to provide deployment actuation and post deployment locking. Hingejoints previously used in moderate precision structures have relied onhigh levels of preload and friction to eliminate freeplay and geometricambiguity. These joints have been shown to be unstable at the micronlevel, causing the structure to “micro-lurch” or change shape and thusmove the instrument's optics far out of alignment.

Existing joints for precision space structures relied on high levels ofpreload between the many components to eliminate gaps and free play thatcause inaccuracies in the structure. Unfortunately, these high levels ofpreload introduce correspondingly high levels of friction both duringthe deployment and after deployment has been completed. Frictionmechanisms are nonlinear and thus are more difficult to control and lesspredictable.

Other hinge designs such as latch and actuator type systems suffer fromthe same disadvantages.

Recently, foldable truss members have been developed so that a trussstructure can be collapsed and compactly packaged to save space duringdelivery and then released to expand and return to its original shape inorbit. All of these mechanisms add to the mass, expense and complexityof the structure and to the difficulty and expense of transporting it.These foldable members reduce the mass (and the delivery cost) of thestructure by replacing the hinge, latch and actuator mechanisms with onesingle device. See, e.g., U.S. Pat. No. 4,334,391 incorporated herein bythis reference.

Solid rods are joined on their ends forming a truss structure (a squareframe for a solar panel array or a superstructure for a communicationssatellite antenna, for example) and pre-selected rods are cut insections to form a hinge between the two sections. The rod sections arejoined with spring steel elements similar to if not actually lengths ofa carpenter's tape measure.

The rod sections can be folded with respect to each other by imparting alocalized buckling force to one of the spring steel elements. Simplyletting go of one rod section, returns the two rod sections to an end toend alignment due to the potential energy stored in the biased springsteel hinge elements.

In this way, a truss structure made up of several of these foldable rodscan be designed on earth, collapsed for delivery to space, and thenreleased once in position in space where the foldable rods flex backinto position forming the truss structure designed and constructed onearth.

In use, this spring steel hinge design suffers from a number ofshortcomings.

First, hinges formed of spring steel elements require joining the endsof each spring steel element to a rod section. These joints and thespring steel elements themselves add significantly to the overall weightof the truss structure which is an undesired factor in space launchcapability.

The spring steel elements also result in dimensionally unstable trussstructures. The dimensional instability is caused by the relative motionof the internal components including the joints between the springelements and the rod sections and permanent yielding of different areasof the spring elements themselves.

The result is that the shape of the truss structure may change when itis erected in space from the shape of the truss structure before it wascollapsed on earth. This can have disastrous effects on instrumentperformance as even a ten nanometer to ten micrometer displacement canseverely affect the performance of primary and secondary optics attachedto the truss structure.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide a foldable memberand a collapsible structure made of a number of foldable members that islighter than prior art foldable members and collapsible structures.

It is a further object of this invention to provide such a member andsuch a structure which is more dimensionally stable.

It is a further object of this invention to provide such a foldablemember which is a single piece design thereby eliminating numeroussources of imprecision.

It is a further object of this invention to provide such a member andsuch a structure which eliminates the need for deployment actuators andmechanical latches to further reduce system mass.

It is a further object of this invention to provide such a member andsuch a structure which have tailorable thermal expansion andconductivity properties and which thus can be designed for a multitudeof performance requirements.

It is a further object of this invention to provide such a member whichcan be made of a variety of different types of materials.

It is a further object of this invention to provide such a member whichis simple to manufacture and use.

It is a further object of this invention to provide a collapsible tubeuseful in variety of applications.

This invention results from the realization that a lighter and moredimensionally stable, foldable member can be constructed by cutting orforming longitudinal slots in a tube around the perimeter thereof at alocation where the member is designed to bend thereby forming separated,longitudinal strips of tube material at that location which easilybuckle allowing the member to fold without adding a separate hinge whichwould add weight to the member which would also result in dimensionalinstability.

The foldable member of this invention includes an integral hinge made ofthe same material that the tube is made of resulting in sharp weightreduction and improved dimensional stability that is especially wellsuited for space applications.

This invention features a foldable member comprising a tube, at leastone predetermined hinge area along the length of the tube, and aplurality of opposing elongated slots in the tube at the hinge areathereof forming separated longitudinal strips of tube material betweenthe slots which fold when subjected to localized buckling forces.

There may be two diametrically opposing elongated slots in the tubeforming two diametrically opposing longitudinal strips or there may bethree opposing elongated slots and three opposing elongated strips, eachlongitudinal strip diametrically opposing an elongated slot.

Typically, there are a plurality of hinge areas longitudinally spacedfrom each other along the length of the tube, each hinge area includingopposing elongated slots.

The tube may be made of plastic (e.g. polycarbonate) material, a metalmaterial, or a composite material such as a triaxial braid of fibers inthe shape of a tube embedded in a resin matrix.

This invention also features a collapsible structure comprising aplurality of joined members. A selected number of these members eachinclude a tube, at least one predetermined hinge area along the lengthof the tube, and a plurality of opposing elongated slots in the tube atthe hinge area thereof forming separated longitudinal strips of tubematerial between the slots which fold when subjected to localizedbuckling forces.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a perspective view of a structure made of a number of foldablemembers in accordance with the subject invention;

FIG. 2 is a schematic view of the structure shown in FIG. 1 in acollapsed state;

FIG. 3 is a perspective view of the structure of FIG. 2 after it expandsfrom the collapsed condition;

FIG. 4 is a front elevational view of a prior art foldable device;

FIG. 5 is a view of the prior art device shown in FIG. 4 in the foldedposition;

FIG. 6 is a side elevational view of the foldable member of the subjectinvention;

FIG. 7 is a front elevational view of the foldable member shown in FIG.6;

FIG. 8 is a schematic view of the foldable member shown in FIGS. 6 and 7in a folded position;

FIG. 9 is a front elevational view of another embodiment of the foldablemember of this invention;

FIG. 10 is a side elevational view of another embodiment of the foldablemember of the subject invention;

FIG. 11 is a view similar to FIG. 11 showing the interior rear side wallof the foldable member of the subject invention;

FIG. 12 is a front elevational view of a single monolithic elongatedfoldable member with multiple hinge areas in accordance with thisinvention; and

FIG. 13 is a schematic view of the member of FIG. 13 in folded position.

FIGS. 14-23 are schematic views of alternative embodiments of foldablemembers in accordance with the subject invention.

Truss structure 10, FIG. 1, of this invention includes a plurality ofjoined truss members 12 and 14 as shown. Truss structure 10, forexample, may be 1.25 meters tall but collapsible to a height of 27centimeters as shown in FIG. 2 due to the foldable nature of trussmember 12 (and other selected truss members) which includes hinge areas16, 18, and 20 along its length.

Depending on its specific design, hinge area 16 may fold downward, hingearea 20 may fold upward, and hinge area 18 may fold in the direction outof the plane of the drawing.

When collapsed as shown in FIG. 2, the volume of truss structure 10 issharply reduced resulting in significant space savings for space flight.

Upon deployment in outer space, however, truss structure 10automatically expands as shown in FIG. 3 to its original configurationand may be used as a frame for solar panels, various optical devices, oras a part of a superstructure when joined to similar structures.

As shown in FIG. 3, the truss structure is strong under compression andcan support a load of at least 200 pounds. Its also strong againstbending and torque since the individual hinge areas can only be actuatedby intentional localized buckling force applied directly to the hingeareas.

In the prior art, the hinges are formed in the truss members by cuttingthe truss members at the desired hinge area and attaching clam shellshaped steel spring elements 40, 42, and 44, FIG. 4 to truss membersections 46 and 48.

The spring steel elements are similar to lengths of carpenter's tapefrom a tape measure. When a localized buckling force is imparted to onespring element as shown at 50 and the two truss member sections aresubjected to a bending force, the spring elements readily bend,collapsing the truss member as shown in FIG. 5. If one truss membersection is released, the clam shell shape of the spring steel elementsspring the truss members into the configuration shown in FIG. 4.

However, these and other such truss members suffer from numerousshortcomings as discussed in the Background of the Invention above,including the fact that they are not thermally stable. Also, the jointsbetween each spring steel element and the truss member sections canshift slightly and/or a spring steel element may yield while the trussstructure is in the collapsed condition. When this truss structure isdeployed in space it may not return to its original shape, resulting indimensional instability which can severely affect the performance ofsensitive equipment and optical devices. Other prior art devices addedsignificantly to the overall weight of the system, were notdimensionally stable, and/or were complex, and/or costly.

In contrast, the subject invention solves these problems in part by amonolithic foldable member with an integral hinge constructed of thesame material as the member. In other words, the member and the meanswhich allow the member to fold or bend are integrated and made out ofthe same material in a single, continuous member. Foldable member 60,FIG. 6, is made of tube 62 having at least one predetermined hinge area64. Hinge area 64 includes opposing, elongated slots 66 and 67 (see FIG.7) forming separate longitudinal strips 70 and 72 of tube materialbetween the slots. These strips 70 and 72 fold when subjected tolocalized buckling forces as shown in FIG. 8, thereby allowing themember to fold at the hinge area about axis 74, FIG. 7. “Slots” as usedherein means openings, slits, and cuts of any configuration.

Because member 60 is constructed of a single, continuous material, it isdimensional stable and extremely reliable. In addition, by tailoring thematerial of tube 62, the thermal expansion and/or conductivity of member60 can be precisely tailored to meet various performance requirements.At the same time, member 60 is sufficiently strong with respect totorsion, shear, and buckling for numerous applications.

Slots 66 and 67, as shown in FIGS. 6 and 7, are diametrically opposingbut this is not a limitation of the present invention. For example, inthe embodiment shown in FIG. 9, there are three opposing elongated slots90, 92, and 94 and three opposing longitudinal strips 96, 98, and 100(see also FIG. 10). Longitudinal strip 96 is diametrically opposed toelongated slot 94, longitudinal strip 98 is diametrically opposed toslot 90 and longitudinal slot 100 diametrically opposes slot 92.Therefore, the slots are spaced around the circumference of the tube ina generally opposing configuration, but a given slot may notdiametrically oppose another slot even if there are only two slots.Also, although the slots are each shown to be of the same construction,this is not a limitation of the present invention as the length andopening width of the slots at a given hinge area may be differentdepending on the specific design. Furthermore, the slots may vary from amere slit to a wide elongated opening. For example, slots 66 and 67,FIGS. 6 and 7, are simply a 4 inch long cut in a 1¾ inch tube. Slots 90,92, and 94, on the other hand, are elliptically shaped and approximately{fraction (11/16)} inches wide at their widest point.

As shown in FIG. 1, a given truss member may include a plurality ofhinge areas such as hinge areas 16, 18, and 20 along the length of trussmember 12. Therefore, any one member may include a number of hingeareas, each hinge area including two or more opposing elongated slots.

Tube 62, FIGS. 6-9 may be made of plastic material such as apolycarbonate material, but polyurethane, Delrin, or nylon tubes mayalso be constructed. Also, for space applications in particular,composite materials may be used including a braided fiber structureembedded in a resin matrix. In one example, carbon fibers were braidedusing a round braider to form a triaxial braid in a tubular shape whichwas then impregnated with a polycarbonate resin. A thin wall aluminumtube was wrapped in Teflon and over wrapped with a sheet of Lexanmaterial. A triaxial carbon braid was formed over the Lexan sheet andadditional layers of Lexan were added over triaxial braid. A combinationof pressure and elevated temperature was used to consolidate the Lexanmaterial into the fibers. The slots were then cut into the tube in thedesired configuration. The tube may also be made of metal.

When structure 10, FIG. 1 was constructed of 1.5 inch diameter tubessimilar to those shown in FIG. 9, it weighed 3.9 lbs. and supported astatic load of more than 200 lbs. This 4 ft. tall structure iscollapsible to an 11 inch tall folded package. Therefore, a 100 footlong structure could be packaged into a “Delta class” space vehicle forspace deployment and would weigh less than 100 lbs. Since material isactually removed from each foldable member when the opposing slots areformed, the resulting structure weighs significantly less than prior artstructures constructed of members including spring steel elements 40,44, and 42, FIG. 4 or prior art structures with mechanical hinges.

In another embodiment, member 120, FIG. 10 includes opposing sets 122and 124 of elongated slots. Thus, set 122 includes two slots, slot 126and slot 128 separated by bridge element 130; and set 124 includes twoslots, slot 132 and slot 134 separated by bridge element 136. Each slotwas about ⅛″ wide and about ⅝″long in a 1⅝ inch diameter Lexan tube.Each bridge element was about {fraction (3/16)} inches long.

In one embodiment, slot 126 is diametrically opposed from slot 132 andslot 128 is diametrically opposed from slot 134, although this is not alimitation of the present invention.

Also, stress relieving member 138 (e.g. a dowel) may be attached to eachbridge element 130 and 136 on the inside of the tube for relieving thestress of each bridge member and to prevent them from tearing orcracking when the tube is folded.

The foldable member shown in FIGS. 10 and 11 proved to be generallystronger in and torsion than the members shown in FIGS. 6-9.

By including the hinges of this invention in a longeron twenty feet inlength, it may be collapsed to a three foot long package, convenient forstorage. A 3-4 inch diameter tube would typically have about a {fraction(1/16)}th inch wall thickness while a 1½ inch diameter tube wouldtypically have a 0.020 inch wall thickness, although many differentcombinations of wall thickness and diameters are possible over a widevariety of tube lengths and tube materials for specific applications.

The result is an integral, monolithic (single material) foldable trussmember, or longeron, or tube with no moving parts or joints and thus alighter and more dimensionally stable structure. The hinge means orelements are made of the same piece of material as the tube unlike thespring steel elements of the prior art.

The members shown in FIGS. 6-11 could be a component of truss structure10, FIG. 1 made of like truss members joined together as shown orinstead could be a longeron of a frame or bulkhead or even a solitaryboom or portion of an arm or other member.

In addition, the members shown in FIGS. 6-11 could be a part of othermechanical structures such as collapsible mobile bridges, erectablecivil engineering structures for emergency response and disaster relief,tent poles, police barricades, and the like.

FIGS. 12 and 13 show foldable structural member 150 with elongated slotsplaced at different locations to allow the member to be folded atdifferent angles of bend to accommodate unique storage and/or deploymentrequirements or sequencing.

Foldable member 200, FIG. 14 is made of a fiber reinforced resin matrixcomposite material which includes embedded therein electrical conductor202 for transmitting electrical signals from one location to another ofthe structure of which member 200 forms a part. Thus, the need forexternal connections and electrical conductors is eliminated.

Foldable member 204, FIG. 15 includes transducers, a shape-memory alloy,or piezoelectric members 206 and 208 proximate hinge area 210 disposedon the outside or the inside of the tubewall of member 204 or embeddedin the wall thereof to control the folding and unfolding of member 204at hinge area 210.

Foldable member 212, FIG. 16 includes elongated slots 214 and 216 andplastic webs 218 and 220 on or stretched partially over the slots on theinside of the tube to reinforce the slots. Webs 218 and 220 may bedisposed wholly across the slots or may include orifices as shown inFIG. 16.

As shown in FIGS. 17-20, the slots need not be in the shape of elongatedovals. In FIG. 17, elongated slots 230 and 232 are shaped like triangleswith the corners rounded. In FIG. 18, slots 234 and 236 are more diamondlike in shape. In FIG. 19 four slots 240, 242, 244 and 246 from anX-pattern of tube material at hinge area 250. In FIG. 20, four ovalshaped slots 252, 254, 256, and 258 form an X-pattern of tube materialat hinge area 260.

Foldable member 280, FIG. 21 includes nested outer tube 282 and innertube 284 for improved stiffness. Slots 286 and 288 are present in thesurface of outer tube 282 and coincident slots 290 and 292 are presentin the surface of inner tube 284. An adhesive may be used to join innertube 284 to outer tube 282.

Foldable member 300, FIG. 22 includes tube 302 made of layers 303, 304,306, etc. of material, plastic (e.g. Lexan), for example, formed bywrapping a sheet of the material around itself several perhaps even 20or more times. An adhesive, for example a double sided tape, may be usedto secure the layers of plastic material to each other at selectedlocations along the length of the tube for example at locations 310 and312, shown in phantom. If the sheet of material comes off a round rollof stock material, it will have a tendency to roll up into a tube due tomemory, an advantageous feature of this embodiment of the subjectinvention.

As with the other embodiments, slot 314 and an opposing slot (not seenin FIG. 24) is formed (e.g. cut) through all of layers 303, 304, and 306forming longitudinal strips of layers of tube material 318 and 320 whichfold when subjected to localized buckling forces. In this embodiment,additional strength is provided by virtue of the many individual columnsof tube material.

In the embodiment shown in FIG. 25, these individual tube layers arelaminated to each other in areas A and B but not at hinge area C. Assuch, the layers of tube material may be made of plastic or compositematerials subjected to conventional lamination processes.

Although specific features of this invention are shown in some drawingsand not others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

What is claimed is:
 1. A foldable member comprising: a tube; at leastone integral predetermined hinge area along the length of the tube; anda plurality of opposing elongated longitudinally running slots in thetube at the hinge area thereof forming separated longitudinal strips oftube material between the slots which fold when subjected to localizedbuckling forces.
 2. The folding member of claim 1 in which there are twodiametrically opposing elongated slots in the tube forming twodiametrically opposing longitudinal strips.
 3. The folding member ofclaim 1 in which there are three opposing elongated slots and threeopposing elongated strips, each longitudinal strip diametricallyopposing an elongated slot.
 4. The foldable member of claim 1 in whichthere are a plurality of hinge areas longitudinally spaced from eachother along the length of the tube, each hinge area including opposingelongated slots.
 5. The foldable member of claim 1 in which the tube ismade of a plastic material.
 6. The foldable member of claim 5 in whichthe tube is made of a polycarbonate material.
 7. The foldable member ofclaim 1 in which the tube is made of a composite material.
 8. Thefoldable member of claim 7 in which the composite material contains atriaxial braid of fibers in a resin matrix.
 9. A collapsible structurecomprising: a plurality of joined truss members; a selected number ofsaid truss members each including: a tube; at least one predeterminedhinge area along the length of the tube; and a plurality of opposingelongated slots in the tube at the hinge area thereof forming separatedlongitudinal strips of tube material between the slots which fold whensubjected to localized buckling forces.
 10. A foldable membercomprising: a monolithic tube; at least one integral predetermined hingearea along the length of the tube; and a plurality of opposinglongitudinally running elongated slots in the tube at the hinge areathereof forming separated longitudinal strips of tube material betweenthe slots which fold when subjected to localized buckling forces andwhich unfold automatically when released.
 11. A foldable membercomprising: a monolithic tube; at least one integral predetermined hingearea along the length of the tube; and elongated spaced'slotscircumferentially disposed in the tube at the hinge area thereof formingseparated longitudinal strips of tube material between the spaced slotswhich fold when subjected to localized buckling forces and which unfoldautomatically when released.
 12. A collapsible structure comprising: aplurality of joined truss members; a selected number of said trussmembers each including: a tube; at least one predetermined hinge areaalong the length of the tube; and a plurality of opposing longitudinallyrunning elongated slots in the tube at the hinge area thereof formingseparated longitudinal strips of tube material between the slots whichfold when subjected to localized buckling forces.